잊어버리다
ijeobeorida · TOPIK 1
to forget completely
잊다 (forget) + 버리다 (throw away) = forget and toss it for good
잊어 is the connective form of 잊다, to forget. 버리다 means to throw away, and as a helper verb it marks an action as finished off completely, often with regret. So 잊어버리다 is forgetting something entirely, not just a brief slip.